This just my feeling now having had the system since launch, and showing it to friends and seeing how they react to it. I think Switch can bring in a "new audience" (for Nintendo), but it's not going to be the typical audience we think of when talking about Nintendo this way, in fact I think it might even surprise Nintendo (just as the PS4's success has surprised Sony in many ways).
I think the "additive" new audience the Switch is actually going to tap into are older 28-40+ year old gamers people who typically would buy a Playstation or XBox. I'm not talking "lapsed gamers" either, I'm talking about people who can game just fine and have all their lives, they're just older now. The person who bought a Playstation at age 16 in 1996 would today be 36/37, life changes a lot in those 20 years.
So why Switch? Because the Switch I'm finding the real draw of its design is that it works well for people who love games but don't have the fucking time to game anymore. When you are past 26/27/28, you have a full time job now, are married usually or in a commited relationship that takes up time, you have kids or kids are coming, etc. etc. etc. You have 1000 things sitting on your PVR that you promised you would watch but haven't gotten around to and another 1000 things on Netflix you've been meaning to get around to.
You love games, but the fact is they are a time consuming and demanding hobby. Enter Switch. No it doesn't have the graphics of a PS4, buuuuut PS3/360 graphics still look decent and can still give you a decent enough game of Skyrim or Zelda or NBA2K or GTAV or MGS. It's such a flexible device that you can get in quick gaming sessions without having to revert back to the 3DS, which lets be honest feels like a toy in comparison to the Switch and doesn't allow for TV gaming (not that you'd want to anyway with that chipset).
I can actually see Nintendo getting 15-20+ million extra gamers from this category ... what typically would be Playstation or XBox territory, but as you get older, real life sets in and you just don't have the time. This audience has a large amount of disposal income and they do love games so there's no hoops Nintendo needs to jump through, what this audience doesn't have is time. They don' have the time to play games like they did when they were 19 years old. I have a PS4 and XB1 but I just don't have time to play them. It just feels like a chore to go down to the game room, start the system up (which usually entails a firmware update of several minutes) and then settle in, and by that time my fiance is usually yelling at me to check on something. Switch on the other hand, I find little pockets of 15-20 minutes where I can play, no problem (yes even on the toilet, I'm not ashamed to say).
You add that extra userbase to what the 3DS + Wii U sold (subtracting 9 million or so for people who bought both) and you are getting pretty close to that magic 100 million number a lot of people here are obsessed with, also consider every new gen brings in a new generation of kids.